[Github] Force changed files step to use GH API for code formatting

This patch forces the code formatting action to use the GH API to get
changed files rather than looking through the git history. This should
save about 10m per job (most of the current runtime) due to the
avoidance of expensive partial clone operations.

Author: boomanaiden154

.github/workflows/pr-code-format.yml

@@ -7,17 +7,20 @@ jobs:
   code_formatter:
     runs-on: ubuntu-latest
     steps:
-      - name: Fetch LLVM sources
-        uses: actions/checkout@v4
-        with:
-          fetch-depth: 2
-
+      # Get changed files before we do a checkout to force the action to use
+      # the GH API to look for changed files and avoid an expensive partial
+      # fetch operation.
       - name: Get changed files
         id: changed-files
         uses: tj-actions/changed-files@v39
         with:
           separator: ","
-          fetch_depth: 100 # Fetches only the last 10 commits
+
+      - name: Fetch LLVM sources
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: ${{ github.event.pull_request.commits }}
+          ref: ${{ github.event.pull_request.head.ref }}
 
       - name: "Listed files"
         run: |